The present invention relates to miniature molded case circuit breakers of the type utilized in residential, office and light industrial applications principally for branch circuit protection and particularly to molded case circuit breakers capable of achieving a current limiting circuit interruption.
As the current availables of electrical utilities are raised, increasing emphasis is being placed on the magnitude of energy let-through a circuit is subjected to when a circuit protective device, such as a circuit breaker, acts to interrupt high fault currents. Energy let-through, expressed by the equation I.sup.2 t, where (I) is current and (t) is time, represents the true damage potential to the circuit and its connected loads associated with a high fault current interruption. It has been proposed that a maximum energy let-through value be adopted as one criteria qualifying circuit breakers for certain circuit applications.
Obviously, the ideal approach to minimizing energy let-through during a high fault current interruption is to limit the current (I). An exceptionally fast acting circuit breaker capable of effecting early contact separation and then developing an arc voltage greater than the system driving voltage such as to crest a fault current wavefront at a level well below its prospective peak amplitude, serves this goal. However, this is not the complete answer, since time (t), the interval from the onset of a fault current wave to final arc extinction, should also be minimized. This requires optimization of the arc chamber and arc chute designs, not only to rapidly develop the requisite high arc voltage, but also to achieve a full and final arc quench as quickly as possible.
It is accordingly an object of the present invention to provide a molded case circuit breaker capable of significantly limiting the let-through energy accompanying a high fault current interruption.
A further object is to provide a molded case circuit breaker of the above character which is capable of true current limiting operation.
Another object is to provide a molded case circuit breaker of the above character having improved arc extinguishing capabilities.
Yet another object is to provide a molded case circuit breaker of the above character which is efficient in construction and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.